Dialing system



W. P. ALBERT DIALING SYSTEM May 16, 1944.

Filed July l, 1942 7 Sheets-Sheet l ATTORNEY May 16, 1944. w. P. ALBERT DIALING SYSTEM Filed July 1, 1942 '7 Sheets-Sheet 2 nu .i

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DIALING SYSTEM Filed July 1, 1942 7 Sheets-Sheet 6 /NVENTOR Y By WRALBERT A T TORNE V May '16, 1944. v w, p; ALBERT 2,349,129

DIALING SYSTEM Filed Julyv 1. 1942 l 7 Sheets-Sheet 7 p; fr; y; l;

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; i i i gma i i sii i i i v OLII. i /NVE/VTOR WPALBERT v @Y Arrow/Ey Patented May 16, 1944 UNITED STATES PATENT OFFICE DIALING SYSTEM Walter I. Albert, Madison, N. J., assignor to Bell .Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application July 1, 1942, Serial No. 449,257

4 Claims.

This invention `relates to communication sy-sy tems and particularly to signaling means therein.

The general object of the invention is to provide fast acting reliable dialing means of the push-button type for transmitting number signals for the lpurpose of controlling switching apparatus in central office installations.

Another object of the invention is to piace under subscriber control only the most simple and rugged apparatus in a metallic line circuit arrangement.

In accordance with these objects a dialing means at a substation is provided in the form of a plurality of push-buttons and a pair of varistors which may thereby be selectively cut into the substation loop in various sequences. A responsive means at the central ofl'lce `end of the line will register the permutations of the various conditions thus set up and act as a switching control means.

The substation means consists generally of a plurality of digital keys for writing up the number of a wanted line in the usual manner. Each key is arranged to open the substation loop and to then insert in the open circuit a nonlinear resistance element spoken of hereafter as a varistor pointed in one or the other direction. This action is repeated on the upstroke of the key so that upon each actuation of a key lthere are four conditions which may be detected and these set in a permutation code will give sixteen selective possibilities. In some respects the 'actuation of the key is analogous to the operation of a teletypewriter key for upon the downstroke it first produces a start signal in the form of an open circuit, follows this with -a connection which may be interpreted four ways and finally produces a stop signal in the form of a closed circuit. Upon the upstroke of the key another start signal in the form of an open circuit 'is produced followed by a circuit condition which may `be interpreted four ways and finally pro-v duces a stop signal in the form of aclosed circuit.

At the other end of the line there are provided a plurality of sets of detecting devices, prefer#- ably inthe form of cold cathode tubes. Upon the receipt of the said start signal responsive apparatus will produce a simulated single full cycle of alternating current which is used -to detect the conditions produced by the actuation of the vkey at the substation. The result of f'- `the test made by the transmission of this single cycle is recorded on cold cathode Itubes and'is later used to control the desired switching operations.

A feature of the invention is the use of a pair of varistors at the controlling point which may be included in the line circuit thereat sequentially and selectively.

Another feature is the method of signaling which consists in sending a start signal fromone end of the line to the other and then selectively presenting a predetermined circuit condition .at said one end to .be tested from thesaid other end by currents generated at saidlother end in response to the receipt of said start signal.

Still another feature of the invention is a means for generating a single simulated cycle of alternating current for testing purposes with means responsive to the strength of each half wave to indicate selectively established circuit conditions at the point to transmitted. i

cluding a set of cold cathode tubes for registering the sequential and selective imposition of certain circuit conditions at a distant point.

Another feature is the use of a push-button signaling device which will set up different circuit conditions on the forward and return strokes thereof.

Another feature is the means for starting two trai-ns of loperations one on .the forward stroke `of the signaling device and one on` the return strokethereof, which trains may be separated in time. .Regardless of such separation in time, such two trains of operations will produce a single result at the receiving end which may be used to register a single item of intelligence as a single digit of a telephone number.

Another feature is signaling by transient cur- The receipt of the start signal results in the generation of, first, a positive transient and, second, a negative transient both aperiodic in character but of sufficient magnitude vto operate as a testing`current which will operate or fail.

to operate a local medium in response to a condition established or not established at the far end of a line. i

Other features will appear in the description.

The drawings consist of seven sheets which may be'combined as shown in Figs. l and 2. Figs. 3, 4 and r5 in Vcombination serve to explain a single digital key operation and the reaction at vthe other end of a line caused thereby. Figs. 3, 4 and 6 to 9 serve to` explain the registra tion of a designation consisting of a plurality of digits. Figs. 3 and 4 are schematic circuit diagrams showing the sequential operations of a following The key arrangement is such that a particular key will set up two independent circuitconditions, one on the down stroke and another on the up stroke. These circuit conditions Amay -be alike or they may be different. 'Ihemechan-` ical arrangement of the spring contacts may be similar to the arrangement shown and described in the application Serial No. 444,772, filed May 21S, 1942 by Bascom and Hubbard.

Each of the'separate circuit diagrams of Figs. 3 and 4 shows'a path through the key contacts from one linewire to another and each atthe time ofits consideration is to be connected to the line wires at the extreme left in Fig. 5. The rstfdiagr'am of 3 is the normal condition before` the -key isdepressed and the last is the normal conditionat the eXtreme endof the down stroke. 'Ihis diagram may also'be considered the normal Vcondition before the start of the up stroke since the subscriber may hold the key in this condition for 'an indefinite period. The 'last diagram in Fig. -4 is also a duplicate of the rst diagram in Fig.' l3 and represents the normal condition attained at the extreme end of the upstroke. The `intermediate diagrams represent various stages in the'sequential operation of the springs, and it is believed that with the help of the notations the operation will be abundantly I clean g y At 4the `top of these diagrams there is 'a line which represents the direct current flowing in the line circuit. This ows at' a value fixed'by the various circuit factors vand remains at a fixed point until the fourth stage is reached when it drops to zero as the circuit is effectively opened at'this point. This condition exists through the fth stage and until the rsixthstage is reached whereupon the circuitfor the'direct line current is again closed. f'Duringthis'interval a Stest curl" rent consistingof a single cycle of alternating current 'is sentl from the distant end to detect the circuit condition established at the substation. Similar operationsy take place on the up stroke ofthe key. y

The essential operation is to sele'ctivelyconnect a pair of varistors in the circuit and to then test from and register at; the other end of the line which of these varistors have been introduced into the circuit. The twol varistors in the circuit diagrams of Figs.` 3 and 4 are marked- A and B, respectively.' If the A Varistor has been included in the circuit, then the positive impulse or the positive half wave of the single cycle of alternating current willv find a path of low lreend of the line will'result inthe registering .of .a

ing that neithervaristor has been connected in the circuit; second, a positive signal, showing that the A' varistor has been connected in the circuit; third, a negative signal, showing that the B varistor has been connected in the circuit, and fourth, both a positive and a negative signal showing that both varistors have been connected in the circuit. Sinceone selective connection may be made from the down stroke and a different one for the up stroke of the key it follows that there are, theoretically, sixteen different avail-able permutations which may be used for signals. Since that one where there is no response in either the down stroke or the up stroke could not be used vfor certain practical reasons it follows that there are fifteen codes which may be vused forl digital designations. Ten of these may be used in regular service and the other five may be used for certain special service features, such as'for calling long distance or for making certain emergency calls, such as to the police or the fire services. The `following table shows the combinations which may be used.

Down stroke Up stroke Digit By Way of example the diagrams of Figs. 8 and Llshovv the key contact operations for the sending of digit 5 according to the above table.

v The essential operations of a key are then to select the varistor to .be used, to open the line as a start signal to the apparatus at the other end 'of the line and then to connect in the se# lected varistor to expose it to the single cycle of alternating current which will be. sent from the other end of the line in response to the said start signal. These operations both for the down stroke and for the up stroke are shown in step# by-step detail in Figs. 3 and ll. The ltime values indicated are by way of example and arevnot 55 necessarily controlling.

The various contacts of the keys at the substation are designated a, b, c, d and e. The two varistors A and B are cut into the loop circuit byclosure of key contacts a and b, respectively,

followed by the transfer ofthe key'from contact d to e. All keycontacts areco-mmon to all keys but contacts a and b are controlled by mechanical cams on the individual keys so that various combinations may beobtained on the down stroke and different combinations on the up stroke of the keys. Contacts c, Yd and e function on each down stroke and again on each up stroke of any key. The order of operation of contacts on the down stroke is: `c opens, a and/or b close if four diierent conditions: rst, no response show- 7 required, d transfers 'to e with a definite open interval, then, near theend of the down stroke, c reclosed, a and b open followed by transfer from etc d.` *Ony the up stroke the same order is required, namely, c opens, a and/or b close, d transfers to e and near the end of the up strokec recloses, a and b open and e transfers back'to d. The transfer from contact d to e on both strokes of any key may be warranted to open the loop by a toggle'sv'vitch arrangement for a denite interval independent of the speed of opera; tion of the key.

The momentary opening of the loop caused by the transfer from contact dto contact e in the subset is the signal for the sender to send out a pulse of positive and negative current to determine the polarity condition existing by virtue of the closed or opened a and b contacts. When the line has been taken for use a circuit is established from ground through the winding of relay I, the tip of the line T through the loop closed at the substation, the ring of the line R and thence through the Winding of relay 2 to battery. The momentary opening of the loop induces in the secondary coil of relay I a potential which is suflicient to energize the tube 3 which thereupon 'locks in its anode circuit to positive battery through the 'windings of relays 4 and 5 .and in series with the primary winding of a transformer having the windings 6, 'I and 8. The build up of ux in the transformer due to current flowing in its primary Winding induces a potentialV in its secondary winding 6 and this is connected so that the terminal which is positive 'on the build up of flux is connected to the tip of the line through the resistance 9. vThis positive potential is therefore applied to lthe tip of the' line and finds the path over the loop .through varistors A 'or B, over Vthe ring of the line through varistor IIL'back to the other terminal of the secondary winding 6. Since varistors A and B are designed with very low resistance in one direction and very high resistance in the opposite direction, there will be a very great difference in the amount of current which will now in the resistance 9.

If varistor A vis connected in the circuit then a relatively large current will flow and the drop in potential across resistance 9 will cause tube I I to fire. The circuit of tube I I includes a resistance I2 and a contact on progress switch controlled by magnet I3 and leading to conductor I4. The drop in potential across the resistance I2 due to current flowing through tube `Il is `added to the voltage supplied to the other' terminal of tube I to bring the total potential Aabove the ring point and tube I5 will re and thereafter remain in an ionized condition by the sustaining battery voltage until a subsequent registering operation extinguishes it. Tube II, .however, is operated only while the positive current is flowing in the resistance 9.-

The progress switch which is under control of the magnet I3 may be of the type shown in the application of Frank H. Hibbard, Serial No. 411.105. filed September 17, 1941.

As previously noted, the positive half of the test cycle is induced in the secondary winding 6 of the transformer by the build up of flux due to currentowing in the primary winding of the transformer by the firing of tube 3. The value of the voltage induced is dependent upon the time rate of change of ilux acting upon the total turns of the secondary Winding. The length of the positive pulse is largely dependent upon the design of the transformer but is also influenced by the load on the secondary and tertiary windings as well as the external inductive elements in 'series with the primary winding, namely, relays 4 and 5'. It may be assumed that the flux in this transformer is almost fully built up and that relays 4 and 5 Vwill operate in approximately .004 second. Relay 5, in operating, closes its contact and shortv circuits itself in series with the primary winding of the transformer and tube 3. This causes the ux in the transformer to decrease ata rate dependent upon the design of the transformer and loads on all three windings. During the collapse of the flux a negative voltage is produced at the terminal of the secondary winding 6r which is connected at the tip side of the `line and a positive potential at the terminal which is connected to the ring side of the line, through resistance I6 and the make contact of relay 4. This negative half lwave of the cycle of test current will be designed to be approximately the same value in potential and of about the same duration as the positive half cycle. If the B vari'stor' at the subset is closed across the loop by Contact b then a relatively large positive current will now -from thefsecondary winding 6 of the transformer through resistance I6, make contact of relay 4, over the ring of the line through varistor B, thence over the tip of the line through the tip' varistor I1, back 'to the other terminal of the secondary winding 6 of the transformer. This produces a drop in voltage across the resistance I6 sufficient to momentarily re tube I8. This, in turn, increases the potentialacross tube I9 sufficient to nre it. This latter tube issustained in an operated condition by the battery eil.' As soon as the lvoltage of the anode of tube I3 drops below the sustaining value it will extinguish and open the circuit for relay 5. .This relay requires 4approximately .004 secondrtol release during `which the rrelay 4 is `held operated. As soon as relay 5 opens its contact relay 4 will release and the circuit is again normal except that the progress switches controlled by the magnets i3 and 2i have progressed to terminals associated with tubes 22 and 23, as will be described hereinafter. The key at the subset will now reach the end of the down stroke and contact c will -reclose the loop circuit. Thekey may be kept depressedfor as long or as short a time as desired.

The return stroke of the digit key at the subscribers station will repeat the same order of operation of key contactslbut the contacts a and b are now actuated by diiierent mechanical cams than on the down stroke. The opening of the loop on the return stroke again fires tube 3 vand repeats the generation of the test current as described for the down stroke. This test current is always positive at the tip rst followed by positive at the ring, resulting in the generation of a simulated full cycle of alternating current. On the up stroke tube 22 will be red if contact a is closed and tube 23 will be iired if contact b is closed. When the varistors at the subset are .connected so they oppose the now of the positive or negative current, then the drop of potential across resistances 9 or IB is of low value and `tubes II or I8 will not re. Hence tubes |15, I9, 2.2 or

23 will not fire.

In the example given in the drawings of Figs. 3 and 4. tube I5 will fire on the down stroke of the key and tube I9 will fail to -fire. On the -up stroke tube 22 will fire but tube 23 will fail toiire.

The means for progressing the registration from down stroke to up stroke of tbe same digit and for progressing from digit to `digit is shown in Fig. 5. Four tubes are required to register a digiti although with the ten-digit ycode used, not more than two out of the four will be energized 4associated with tube 23.

for any'given digit. vThe fcontacts of theiprogress switches are normally rset-.to render' the circuit for tubes I5 andIS leffective when the sender is seized for an incoming-call. Whenthe rst digit key is operatedl itl opens the loopf-onthe down stroke and causes the transformer to induce a cycle'of alternating test current inl its secondary winding 6, as previously described. At the same time a corresponding cycle of alternating current is induced in the tertiary winding 8 of this transformer for the purpose of advancing the progress switches controlled by the magnets I3 and 2|. These switches are arranged to have the required number of positions for the total number of digits to be keyed and are capable of high-speed operation. When the positive half cycle of the line test current is about at its maximum and ready to fire tubes II and I5 (assumingrv they are' to be actuated) the voltage acrossfthe tertiary winding-8 of the transformer has reached a point Wheretube 24 will re on its cathode circuit from the potential drop across resistance 25. The tertiary winding 8 is connected so that the terminal which is positive when the flux is building up is connected to tube 24. Varistor125 short circuits resistance 21 for this positive current but varistor 28 compels the current to flowfthrough resistance 25, causing tube 24 to operate. Tube 24 being fired, locks in its anode circuit from positivebattery through magnet I3 and its self-interrupting contact. The magnet of this selector therefore starts to energize at almost the same instant thatthe registration' 4of the test current-is being made by tube I5. By the time the selector magnet I3 has energized to the point of breaking its contact with the terminal connected to tube I5, the register tube will have-been actuated and the test with the negative half cycle will be in progress. When the selector magnet. I3 is fully energized it breaks its interrupter contact, thus opening the circuit for tube 24 which is extinguished and selector magnet I3v releases to render the circuit for tube 22 now effective.I Since this tube is not actuated until the positive test on the up stroke of the key there `is, ample time for the operation of .the selector magnet I3. Likewise, while tube I9 is taking the registration of the negative half wave of the test current, the corresponding negative voltage acrossy the tertiary winding!! of the.` transformer will re tube 29 which locks in its anode circuit through selector magnet 2l. Selector magnet 2I operates and releases in a manner similar to the selector mag-r net I3 and advances. its contact to the terminal It should be noted that with the proper design of the transformer there can be practically no variation between the tuberegistration lresulting from voltage `inducedin .the secondary winding and the energizing of the selectors for advancing resulting from. a corresponding voltage induced in the tertiary winding of the same transformer. Because of this close timing it is possible to start energizing the selector magnet while the registration is still in progress.. This cycle of oper.- ation is completed for the down and up stroke of each digit key. For a seven-digit` number there must be fourteen steps of both selectors.

The drawings arranged as indicated in Fig. 2 show the translation and registration transfer circuit which functions to test the four tubes for each digit simultaneously and to operate corresponding detector and translating relays which convert the code toa decimal system for final registration (by way: of example) on. a crossbar register switch. It should be noted that the various elements in Figs. A6A and 7 which correspond to similar elements in Fig. 5 are numbered in the same manner. Thus the two progress switching magnets of Figs. 6 and 'I are designated I3 and 2I, respectively, `to correspond: to the same magnets in Fig. 5. Each digit is tested, translated and registered, one at a time,`follow ing the temporary `registration on the tubes. The progress switch controlledv by thefmagnet 30 has six brushes 3I'to 35, inclusive. The selector has a normal position with lthe lfollowing seven terminals of the banks: controlled by brushes 33, 34, 35 and 36 wired to the tube register'circuit. Means is provided so that this progress switch cannot advance `to a tube to register a digit until theA progress lswitch controlled by the 'magnet 2! has advanced from that digit.` The transfer of the registration starts as soon as the rst digitv is completely registered on the tubes. Off-normal ground is connected to the armature of magnet 30' as soon as the circuit is seized for use and atA the same time the same oli-normal ground is connected to the lower seven contacts controlled by magnet 2|. This off-normal ground, which isindicated by a dotted line, causes relay 31 vto operate through the normal contact of the selector switch brush 32 in a circuit which may be traced from the off-normal ground armature and break contact of magnet 30, normal brush contact of the bank operating with brush 32, through brush 32, the normally closedcontacts controlled by armature I of relay 38, through the upper wind'- ing of relay 31 to battery.I Another circuit may also be traced from the off-normal ground to conductor 39, through the rst or second pair of contacts controlled by magnet 2I, through the normal contact of the bank controlled by wiper 3| to supply ground-to the same point as the armature and -back contact of magnet 30. -Relay 31, in operating, energizes relay 38 which locks to the double source of off-normal ground just traced and by a mechanical sequence of contacts opens the circuit for relay 31 which yimmediately releases since no select magnet is energized. Relay 38, operated; energizes magnet 3B -in a circuit which may be traced' from battery winding of magnet 30,- front contact and armature 2 of relay 3B and thence in a series circuit through the back contact and armature 6 of relay, back contact and armature 5 of relay 4I, back contact and armature 4 of relay 42, back contact and armatures 3 of relay 43 to ground. This checks the fact that the four translation relays 40 to 43, inclusive are released. If this circuit is complete the magnet 34 will operate and open one path of off-normal ground which is holding relay 38 locked. But the other off-normal ground over the contacts controlled by stepping magnet 2| is not removed until this switchhas left the terminal associated withthe tube 23 which indicates nal tube registration of the rst digit. l

Relay 38 Ais slightly slow to release so it will not release on the short opening of the off-normal ground when the selector switch controlled by magnet 2I advances from tube I9 to tube 23. Therefore, as soon as tube registration of the first key digit is completed relay 38 yis released which releases the magnet 3,0, allowing it to advance its brushes to position I. In this position brushes 33, 34, 35 and 36 are connected through the bank to the four tubes representing digit 1, namely, tubes I5,y I9, 22 and 23. Four detectorrelays 44-.to 41 are connected simultaneously-to the tube register circuit in such an arrangement as will determine which tubes are fired. There will be a drop in potential across the associated resistance sulicient to operate the detector relay connected lacross it,y

through a common resistance 48 which will prevent excessive current Aflowing in the register tube circuit. The operationof the detector relay willcause it to lock tothe positive side of the sustaining battery 28, thus short circui-ting the register tube and extinguishing it. The detector relay or relays (normally not more than 2) will operate the corresponding translation relay or relays 4,8 to 43 which in turn unlocks the associated detector relay. The translation relays lock to ground at the back contact and armature 3 ofrelay 38. The operation of the translation relays will connect ground to the proper select magnet representing the decimal of the digit key according. to the` code shown in a previous paragraph. The operation of a translation relay 40 to 43 will release the associated detector relay. The operation of a select magnet closes ground through the primary winding of relay 31, closed contact of relay 38, brush 32 to the hold magnet for the rst digit.

Since it was assumed that the digit was being registered which would mean that tubes I5 and 22 were red, then detector relays 44 and 46 would have operated, resulting in the operation of translator relays 43 and 4 I Therefore a circuit can be traced from ground, armature 3 and front contact of relay 43, armature 3 and back contact of relay 42, armature 3 and front contact of relay 4 I, Winding of select magnet 48 to battery. Select magnet 48 prepares the contacts in its horizontal row for operation and at the same time closes a circuit through its armature vand front contact to extend ground over the lower winding of relay 31, thence through the normally closed contacts con trolled by armature I of relay 38, wiper now on its first position contact to the winding of hold magnet 48 and thence to battery. Hold magnet 49 now causes the contacts at 58 to operate to register 5 as the first digit. Relay 31 is differential and will not operate until locking ground is returned by the operation of the hold magnet. The operation of relay 31 then causes relay 38 to operate and lock to the off-normal ground through the back contact of magnet 38 and to the oit-normal ground through the next pair of contacts controlled by the magnet 2| and associated with the tubes for the second digit if this switch has not advanced from digit 2. Assuming digit 2 has been keyed and the latter path is open, the operation of relay 38 opens the ground at the back contact of its armature 3 which releases the translation relays. The release of the translation relays opens the circuit for the select magnet 48 and closes a back contact circuit through armature 2 of realy 38 to operate the magnet 30 of the selector switch. Relay 38 will release when the select magnet 48 has released, thus releasing relay 31, when the selector magnet operates to break its back contact and when the selector switch controlled by magnet 2 I has advanced from the registration of the second digit. The lead through the armature 2 and front contact of relay 38 again checks that all translation relays are released. Relay 38 releases and causes selector magnet 38 to release, which advances the registration transfer circuit to the tube register circuit for digit 2 and connects to the second hold magnet. This operation is repeated for each digit. If at any time the transfer circuit represented by the selector of Fig. 8 catches up to the next preceding tube register circuit connected to by the switch controlled by magnet 2Irelay 38v willbe held operated, preventing the advance of .the` selector of Fig. 8- until the switch controlled by magnet 2l indicates by advancing that thegsucceeding digit has been registereduon the tubes. All register tubes are extinguished as the transfer circuit advances. The registration on the crossbar switch is then used in `conjunction'witharnarker to set up thecall in the usual manner. Y

PartyV line id'entiiicationmay be obtained by the `additionto the central cnice of four register tubes precedingVJ the first digit and the requirement that party-line subscribers ,operate an added start key at vthe subsetrbefore keying the called number. This added key, -ca r i be arranged to *use digits 11, 1.2, 13I or 14` of the code table hereinbefore setV forth, dependent uponk which party .originates the call. :These particular four codes arelthe'only onesv requiring `the operation of three registerA tubesforeach `codeso that an identifying check can be made of the party codes. In a similar manner digit 15 of the code table can be used by an added key to send a special code for use on long-distance calls.

What is claimed is:

1. In a signaling system, a line, a plurality of differently poled varistors at one end of said line,

a plurality of keys thereat, each said key having means constructed and arranged on its operating stroke to transiently open said line circuit and to then selectively insert therein said varistors in one combination and on its return stroke to transiently open said line circuit and to then selectively insert therein said varistors in the same or another combination, and means at the other end of said line responsive to an open circuit condition established at said one end to transmit testing current to said one end for determining the presence and polarity of said selectively inserted varistors and to register at said other end the nature of the connection established by the said selective insertion of said varistors in said line, said means responding to both the operating and return stroke operations of said keys.

2. In a signaling system, a line, a plurality of diierently poled varistors at one end of said line, a plurality of keys thereat, each said key having means constructed and arranged on its operating stroke to transiently open said line circuit and to then selectively insert therein said varistors in one combination and on its return stroke to transiently open said line circuit and to then selectively insert therein said varistors in the same or another combination, and means at the other end of said line responsive to an open circuit condition established at said one end to transmit testing current to said one end for determining the presence and polarity of said selectively inserted varistors and to register at said other end the nature of the connection established by the said selective insertion of said varistors in said line, said means responding to both the operating and return stroke operations of said keys, said means comprising a set of four cold cathode tubes for each complete key operation, whereby information may be recorded in a four place permutation code.

3. In a signaling system, a line, a plurality of differently poled varistors at one end of said line, a plurality of keys thereat, each said key having means constructed and arranged on its operating stroke to transiently open said line circuit and to then selectively insert therein said varistors in one combination and on its return stroke to transiently open said line circuit and to then selectively insertltherein said varistors in the same or another combination, means at the other end of said line responsive to an open circuit condition established at said one end to transmit testing current to said one end and to register at said other end the nature of the connection established by the said selective insertion of said varistors in said line, said means responding to both the operating and return stroke operation of said key, registering means for each stroke operation of said key, and distributing means for effectively and sequentially introducing said registering means in circuit controlled by said test current transmitting means. i

4. In a signaling system, a line, a plurality of differently poled varistors at one end of said line, means at said one end of said line for transiently opening said line and then selectively closing said open circuit through said varistors, means at the other end of said line comprising an inductance coil having a rst winding in series with a source of direct current and a second winding coupled with a transient current responsive means responsive to said transient opening of said line for generating a test current comprising a simulated full cycle of alternating current, saidtransient current responsive means comprising -a cold cathode tube responsive to the transient current generated in said second winding of said inductance coil by the transient opening of the circuit of said iirst winding, a transformer having a rs't winding energizedby said cold cathode tube, a second Winding for transmitting said test current and a third winding for generating a con` trol current, a plurality of registers responsive to the reaction of said test currents to said diierent line conditions established at saidl one end of said line and a progress switch operated by said control-currents for sequentially rendering said registers effective,

WALTER P. ALBERT. 

